Enhancing Energy Storage Performance of 0.85Bi0.5Na0.5TiO3-0.15LaFeO3 Lead-Free Ferroelectric Ceramics via Buried Sintering

Bismuth sodium titanate (Bi0.5Na0.5TiO3, BNT) ceramics are expected to replace traditional lead-based materials because of their excellent ferroelectric and piezoelectric characteristics, and they are widely used in the industrial, military, and medical fields. However, BNT ceramics have a low breakdown field strength, which leads to unsatisfactory energy storage performance. In this work, 0.85Bi0.5Na0.5TiO3-0.15LaFeO3 ceramics are prepared by the traditional high-temperature solid-phase reaction method, and their energy storage performance is greatly enhanced by improving the process of buried sintering. The results show that the buried sintering method can inhibit the formation of oxygen vacancy, reduce the volatilization of Bi2O3, and greatly improve the breakdown field strength of the ceramics so that the energy storage performance can be significantly enhanced. The breakdown field strength increases from 210 kV/cm to 310 kV/cm, and the energy storage density increases from 1.759 J/cm3 to 4.923 J/cm3. In addition, the energy storage density and energy storage efficiency of these ceramics have good frequency stability and temperature stability. In this study, the excellent energy storage performance of the ceramics prepared by the buried sintering method provides an effective idea for the design of lead-free ferroelectric ceramics with high energy storage performance and greatly expands its application field.


Introduction
Ferroelectric materials are widely used in the detection, conversion, processing, and storage of various kinds of information because of their excellent ferroelectric and piezoelectric properties [1][2][3][4][5].However, traditional ferroelectric ceramics contain the Pb element, which will cause serious harm to the human body and the environment in the production, use, and waste of ceramics [6,7].Among many lead-free ferroelectric materials, because Bi 2+ and Pb 2+ have a similar single-pair electron 6 S 2 structure, Bi 0.5 Na 0.5 TiO 3 (BNT) ceramic has excellent ferroelectric properties [8].BNT-based ceramics are considered to be one of the most likely to replace lead-free ferroelectric ceramics [9,10].However, the low breakdown electric field strength and poor energy storage performance of pure BNT ceramic limit its application in electrical fields [11].In order to improve the electrical properties of BNT ceramics, the modification of BNT-based ferroelectric ceramics has become the main research direction in the ferroelectric field.Some of these new BNT-based ferroelectric materials are already well known.For example, Jiang et al. [12] constructed 0.8Bi 0.5 Na 0.5 TiO 3 -0.2Ba0.3 Sr 0.4 TiO 3 and mixed 0.1NaNbO 3 on this basis to obtain a good solid solution structure.A high energy storage density of 2.26 J/cm 3 at 180 kV/cm was obtained, and the relaxation characteristics and dielectric temperature stability of the material were enhanced.Guo et al. [13] engineered and synthesized (1−x)(0.94Bi0.5 Na 0.5 TiO 3 -0.06BaTiO 3 )-xBiMg 2/3 Nb 1/3 O 3 solid solution to achieve the co-existence of tetragonal-and rhombohedral-phase PNRs in the perovskite structure, and an ultra-high energy density of 6.3 J/cm 3 and an energy efficiency of 79.6% were obtained.Zhang et al. [14] found that the La element can improve the energy storage density and efficiency of 0.93 (Bi 0.5 Na 0.5 )TiO 3 -0.07Ba(Ti 0.945 Zr 0.055 )O 3 ceramics.Gong et al. [15] found that Bi 0.9 La 0.1 FeO 3 ceramics have a low leakage current density and high saturation polarization.At present, a large number of research works have made certain progress, such as controlling the sintering temperature [16,17], using a N 2 atmosphere for sintering [18], adding the sintering aid CuO [19,20], and other methods.Improving the preparation process, such as the sintering process, is also expected to improve the performance of ceramics.Buried sintering refers to when the ceramic structure is buried under something (powder, etc.) for sintering [21,22].For example, Fujii et al. [23] found that sintering BaTiO 3 -Bi(Mg 1/2 Ti 1/2 )O 3 -BiFeO 3 ceramic samples in a bismuth-rich atmosphere, that is, sintering in a closed calcination powder crucible with the same composition, inhibited the volatilization of Bi 2 O 3 , thus enhancing the electrical properties of the ceramics.Li et al. [24] prepared Li 3+x Mg 2 NbO 6 ceramics with excellent microwave dielectric properties using the buried powder sintering method.The purpose of the buried sintering method is to prevent the material loss, volatilization, and combustion of the ceramic at a high temperature during sintering, resulting in the loss of bonding and disintegration of the billet and the inability to maintain its shape.Secondly, buried sintering can have the effect of isolating the air, which can prevent the reaction of the ceramic powder and the components in the air at high temperatures.
Therefore, in this study, 0.85Bi 0.5 Na 0.5 TiO 3 -0.15LaFeO 3 (BNT-LFO) binary solid solution is constructed to study the relationship between its microstructure and electrical properties.More importantly, compared with ordinary sintering, the micro-structure of ceramics is optimized by using buried sintering, which greatly improves the energy storage properties of BNT-LFO ceramics.Besides doping modification, it provides a new design idea for improving the energy storage performance of BNT-based ceramics.

Materials and Methods
BNT-LFO ceramics are prepared by the high-temperature solid reaction method.The experimental raw materials are Bi 2 O 3 (99%), Na 2 CO 3 (99.9%),TiO 2 (99%), La 2 O 3 (99.99%),and Fe 2 O 3 (99.9%).The experimental steps are as follows: According to the stoichiometric ratio, ball milling is carried out for 8 h.After drying, the pre-burning temperature is 650 • C, heat preservation is carried out for 4 h, and the heating rate is 3 • C/min.Next, 5%PVA is used for granulation, and then tablet pressing is carried out (applied pressure: 0.6 Pa, ceramic area: 12.56 mm 2 , and thickness: 0.3 mm).The heat is kept at 650 • C for 3 h to discharge the glue (PVA), and then it is raised to 1100 • C for 2 h for sintering, and the heating rate is 3 • C/min, and then it is naturally cooled to room temperature.The powder used for buried sintering is made of pre-fired powder.The crystal structures of the ceramic samples are analyzed by X-ray diffractometry (XRD, D8 ADVANCE) with Cu K α1 radiation (λ = 1.5406Å).The surface morphology of the samples is characterized by scanning electron microscopy (SEM, JEOL JEM-7800F, Akishima, Japan).The dielectric test system (Wayne Kerr 6500B, Shenzhen, China) is used to test the dielectric properties of ceramics at room temperature.The ferroelectric and energy storage properties of ceramics are tested by a ferroelectric analysis facility (TF-3000, aixACCT, Aachen, Germany) at room temperature, and variable temperature energy storage is carried out at 20-100 • C.

Results and Discussion
The XRD patterns of BNT-LFO ceramics under ordinary sintering and buried sintering are shown in Figure 1a.All ceramics have a pure perovskite structure without introducing the second phase, indicating that LFO has completely entered into the BNT lattice, forming a good solid solution.The main diffraction peak (110) is amplified, as shown in Figure 1b, and it is found that the diffraction peak position of the buried sintered ceramic is shifted to Materials 2024, 17, 4019 3 of 11 a lower angle compared with that of the ordinary sintered ceramic.According to the Bragg equation, the spacing between the crystal faces increases.Buried sintering can reduce the evaporation of Bi 2 O 3 and the generation of oxygen vacancy, and the smaller the vacancy, the larger the crystal plane spacing.Therefore, the distance between the ceramic crystal faces after buried sintering is larger, and the position of the main diffraction peak is slightly shifted to the left side.In addition, the bottom of the XRD diffraction peak of the buried sintered ceramic shows a wider peak, which is caused by the reduction in the grain size of the ceramic after buried sintering.

Results and Discussion
The XRD patterns of BNT-LFO ceramics under ordinary sintering and buried sintering are shown in Figure 1a.All ceramics have a pure perovskite structure without introducing the second phase, indicating that LFO has completely entered into the BNT lattice, forming a good solid solution.The main diffraction peak (110) is amplified, as shown in Figure 1b, and it is found that the diffraction peak position of the buried sintered ceramic is shifted to a lower angle compared with that of the ordinary sintered ceramic.According to the Bragg equation, the spacing between the crystal faces increases.Buried sintering can reduce the evaporation of Bi2O3 and the generation of oxygen vacancy, and the smaller the vacancy, the larger the crystal plane spacing.Therefore, the distance between the ceramic crystal faces after buried sintering is larger, and the position of the main diffraction peak is slightly shifted to the left side.In addition, the bottom of the XRD diffraction peak of the buried sintered ceramic shows a wider peak, which is caused by the reduction in the grain size of the ceramic after buried sintering.The surface microscopic morphologies of the ordinary sintered and buried sintered BNT-LFO ceramics are shown in Figure 2. The insets are the grain distribution data obtained by using the Nano Measurer (2009) software and origin (2018) software.The mean grain size of the buried sintered ceramic is 1.28 µm, which is significantly lower than that of the ordinary sintered ceramic (1.50 µm), showing a reduction of 14.7%.In addition, the grain size distribution of the ceramics prepared by the buried sintering method is more concentrated, the grain size is more uniform, and the number of abnormally coarse grains is significantly reduced, which is expected to improve the electrical properties of BNT-LFO ferroelectric ceramics.The surface microscopic morphologies of the ordinary sintered and buried sintered BNT-LFO ceramics are shown in Figure 2. The insets are the grain distribution data obtained by using the Nano Measurer (2009) software and origin (2018) software.The mean grain size of the buried sintered ceramic is 1.28 µm, which is significantly lower than that of the ordinary sintered ceramic (1.50 µm), showing a reduction of 14.7%.In addition, the grain size distribution of the ceramics prepared by the buried sintering method is more concentrated, the grain size is more uniform, and the number of abnormally coarse grains is significantly reduced, which is expected to improve the electrical properties of BNT-LFO ferroelectric ceramics.
The dielectric constant (ε r ) and dielectric loss (tanδ) curves of the ordinary sintered and buried sintered BNT-BFO ceramics are shown in Figure 3.The measurement frequencies are 1 kHz, 10 kHz, 100 kHz, and 1 MHz.Two dielectric anomaly peaks can be found, where T s occurs at lower temperatures (~100 • C), which is believed to be characteristic of relaxed ferroelectrics and is associated with thermal relaxation in the PNRs.T m occurs at higher temperatures (~380 • C), which corresponds to the transition from the rhombohedral phase to the tetragonal phase [25][26][27].The ε r of the buried sintered ceramic decreased, mainly because this method inhibits the volatilization of Bi 2 O 3 , which causes charge fluctuation.After the ceramic undergoes buried sintering, the tanδ is reduced, which can effectively reduce the energy loss, providing the basis for obtaining excellent electrical properties.The dielectric constant (εr) and dielectric loss (tanδ) curves of the ordinary sintered and buried sintered BNT-BFO ceramics are shown in Figure 3.The measurement frequencies are 1 kHz, 10 kHz, 100 kHz, and 1 MHz.Two dielectric anomaly peaks can be found, where Ts occurs at lower temperatures (~100℃), which is believed to be characteristic of relaxed ferroelectrics and is associated with thermal relaxation in the PNRs.Tm occurs at higher temperatures (~380 °C), which corresponds to the transition from the rhombohedral phase to the tetragonal phase [25][26][27].The εr of the buried sintered ceramic decreased, mainly because this method inhibits the volatilization of Bi2O3, which causes charge fluctuation.After the ceramic undergoes buried sintering, the tanδ is reduced, which can effectively reduce the energy loss, providing the basis for obtaining excellent electrical properties.At room temperature, the bipolar P-E loops of the ceramics under different electric fields are shown in Figure 4a,b.The bipolar P-E loop of the ceramics under a 150 KV/cm electric field is summarized in Figure 4c.The ferroelectric property of the buried sintered ceramic is obviously better than that of the ordinary sintered one.The key parameters, maximum polarization (Pmax), remanent polarization (Pr), and ΔP (Pmax-Pr) are summarized, as shown in Figure 4d.By comparison, it is found that the Pmax value increased from 12.422 µC/cm 2 to 13.793 µC/cm 2 , and the Pr value decreased from 0.880 µC/cm 2 to 0.708 µC/cm 2 for the buried sintered BNT-LFO ceramics.ΔP increased from 11.542 µC/cm 2 to 13.086 µC/cm 2 .The increase in ΔP is expected to improve the energy storage performance of the ceramics.The dielectric constant (εr) and dielectric loss (tanδ) curves of the ordinary sintered and buried sintered BNT-BFO ceramics are shown in Figure 3.The measurement frequencies are 1 kHz, 10 kHz, 100 kHz, and 1 MHz.Two dielectric anomaly peaks can be found, where Ts occurs at lower temperatures (~100℃), which is believed to be characteristic of relaxed ferroelectrics and is associated with thermal relaxation in the PNRs.Tm occurs at higher temperatures (~380 °C), which corresponds to the transition from the rhombohedral phase to the tetragonal phase [25][26][27].The εr of the buried sintered ceramic decreased, mainly because this method inhibits the volatilization of Bi2O3, which causes charge fluctuation.After the ceramic undergoes buried sintering, the tanδ is reduced, which can effectively reduce the energy loss, providing the basis for obtaining excellent electrical properties.At room temperature, the bipolar P-E loops of the ceramics under different electric fields are shown in Figure 4a,b.The bipolar P-E loop of the ceramics under a 150 KV/cm electric field is summarized in Figure 4c.The ferroelectric property of the buried sintered ceramic is obviously better than that of the ordinary sintered one.The key parameters, maximum polarization (Pmax), remanent polarization (Pr), and ΔP (Pmax-Pr) are summarized, as shown in Figure 4d.By comparison, it is found that the Pmax value increased from 12.422 µC/cm 2 to 13.793 µC/cm 2 , and the Pr value decreased from 0.880 µC/cm 2 to 0.708 µC/cm 2 for the buried sintered BNT-LFO ceramics.ΔP increased from 11.542 µC/cm 2 to 13.086 µC/cm 2 .The increase in ΔP is expected to improve the energy storage performance of the ceramics.At room temperature, the bipolar P-E loops of the ceramics under different electric fields are shown in Figure 4a,b.The bipolar P-E loop of the ceramics under a 150 KV/cm electric field is summarized in Figure 4c.The ferroelectric property of the buried sintered ceramic is obviously better than that of the ordinary sintered one.The key parameters, maximum polarization (P max ), remanent polarization (P r ), and ∆P (P max − P r ) are summarized, as shown in Figure 4d.By comparison, it is found that the P max value increased from 12.422 µC/cm 2 to 13.793 µC/cm 2 , and the P r value decreased from 0.880 µC/cm 2 to 0.708 µC/cm 2 for the buried sintered BNT-LFO ceramics.∆P increased from 11.542 µC/cm 2 to 13.086 µC/cm 2 .The increase in ∆P is expected to improve the energy storage performance of the ceramics.
In order to explore the influence of the buried sintering method on the energy storage performance of BNT-LFO ceramics, we tested the unipolar P-E loops of ordinary sintered and buried sintered ceramics, and the test frequency was 10 Hz.The test results are shown in Figure 5a,b, respectively.The breakdown electric field strength of the buried sintered ceramic is up to 310 kV/cm, which is 1.48 times as large as that of the ordinary sintered ceramic (210 kV/cm).The unipolar P-E loops of the ordinary sintered and buried sintered ceramics under their maximum electric field is shown in Figure 5c.P max increases from 18.741 µC/cm 2 (ordinary sintering) to 36.968 µC/cm 2 (buried sintering), which is an increase of 97.3%.According to the unipolar P-E loops, the discharge energy storage density (W rec ) and energy storage efficiency (η) are calculated.The total energy storage density (W), W rec , and η of the ferroelectric materials are calculated as follows [28]: In order to explore the influence of the buried sintering method on the energy storage performance of BNT-LFO ceramics, we tested the unipolar P-E loops of ordinary sintered and buried sintered ceramics, and the test frequency was 10 Hz.The test results are shown in Figure 5a,b, respectively.The breakdown electric field strength of the buried sintered ceramic is up to 310 kV/cm, which is 1.48 times as large as that of the ordinary sintered ceramic (210 kV/cm).The unipolar P-E loops of the ordinary sintered and buried sintered ceramics under their maximum electric field is shown in Figure 5c.Pmax increases from 18.741 µC/cm 2 (ordinary sintering) to 36.968 µC/cm 2 (buried sintering), which is an increase of 97.3%.According to the unipolar P-E loops, the discharge energy storage density (Wrec) and energy storage efficiency (η) are calculated.The total energy storage density (W), Wrec, and η of the ferroelectric materials are calculated as follows [28]: The Wrec and η values of the ordinary sintered and buried sintered ceramics are shown in Figure 5d.After buried sintering, the Wrec of the BNT-LFO ceramics can be increased from 1.759 J/cm 3 to 4.923 J/cm 3 , which is a huge increase of 179.9%.The η decreased The W rec and η values of the ordinary sintered and buried sintered ceramics are shown in Figure 5d.After buried sintering, the W rec of the BNT-LFO ceramics can be increased from 1.759 J/cm 3 to 4.923 J/cm 3 , which is a huge increase of 179.9%.The η decreased slightly from 81.8% to 77.4%, which is a decrease of about 5.7%.The microstructure of the buried sintered BNT-LFO ceramic is more uniform and compact, and the average grain size decreases, so it shows a larger breakdown electric field strength than ordinary ceramic.The W rec and η values of the ceramics are improved.This result shows that BNT-LFO ceramics can be changed by buried sintering to obtain better energy storage performance, which greatly expands its application in the field of electricity.
The W rec of buried sintered BNT-LFO ceramic is compared with other previously published lead-free BNT-based ceramics, as shown in Figure 6  .The energy storage ability is compared between this work and other previously published lead-free BNT-based ceramics, and the results are shown in Table 1.The breakdown electric field strength of ceramic can reach 310 kV/cm, and the W rec is as high as 4.923 J/cm 3 .The results show that the buried sintering progress is a good strategy for improving the energy storage performance of lead-free ferroelectric ceramics.
buried sintered BNT-LFO ceramic is more uniform and compact, and the average grain size decreases, so it shows a larger breakdown electric field strength than ordinary ceramic.The Wrec and η values of the ceramics are improved.This result shows that BNT-LFO ceramics can be changed by buried sintering to obtain better energy storage performance, which greatly expands its application in the field of electricity.The Wrec of buried sintered BNT-LFO ceramic is compared with other previously published lead-free BNT-based ceramics, as shown in Figure 6  .The energy storage ability is compared between this work and other previously published lead-free BNTbased ceramics, and the results are shown in Table 1.The breakdown electric field strength of ceramic can reach 310 kV/cm, and the Wrec is as high as 4.923 J/cm 3 .The results show that the buried sintering progress is a good strategy for improving the energy storage performance of lead-free ferroelectric ceramics.Excellent temperature stability and frequency stability are the prerequisites for the material to ensure the stable operation of the device in practical applications.Therefore, we test the energy storage temperature stability and frequency stability of the buried sintered BNT-LFO ceramic.Figure 7a shows the unipolar P-E loops of the ceramic at different temperatures.The change curves of W rec and η under the electric field of 150 kV/cm with the increase in temperature are shown in Figure 7b.In the temperature range of 30 • C to 100 • C, the fluctuation in W rec is less than 2.7%, and the fluctuation in η is less than 13.0%.Figure 7c shows the unipolar P-E loops of the ceramic at different frequencies.The change curves of W rec and η under the electric field of 150 kV/cm with the increase in frequency are shown in Figure 7d.The fluctuations in W rec and η are less than 3.5% and 9.5%, respectively.The results show that the ceramics have excellent energy storage temperature stability and frequency stability.
100 °C, the fluctuation in Wrec is less than 2.7%, and the fluctuation in η is less than 13.0%.Figure 7c shows the unipolar P-E loops of the ceramic at different frequencies.The change curves of Wrec and η under the electric field of 150 kV/cm with the increase in frequency are shown in Figure 7d.The fluctuations in Wrec and η are less than 3.5% and 9.5%, respectively.The results show that the ceramics have excellent energy storage temperature stability and frequency stability.

Conclusions
In summary, BNT-LFO ceramics are prepared by the traditional high-temperature solid state reaction method, and the ceramics are made into porcelain by ordinary sintering and buried sintering.The microstructure and dielectric, ferroelectric, and energy storage properties of the ceramics are studied.The results show that buried sintered ceramic can inhibit the volatilization of Bi2O3, reduce the generation of oxygen vacancy, make the microstructure more uniform, and reduce the main grain size.The breakdown electric field strength increases from 210 kV/cm to 310 kV/cm.Wrec increases from 1.759 J/cm 3 to 4.923 J/cm 3 , which is an increase of 179.9%.It shows good stability in the energy storage temperature and frequency.In this study, the advantages of buried sintered ceramic from the microscopic level to electrical properties are deeply discussed, which provides a new method for further improving the energy storage performance of BNT-based lead-free ferroelectric ceramics and lays a theoretical foundation for the development of BNT-based lead-free ferroelectric ceramics with high storage performance.

Conclusions
In summary, BNT-LFO ceramics are prepared by the traditional high-temperature solid state reaction method, and the ceramics are made into porcelain by ordinary sintering and buried sintering.The microstructure and dielectric, ferroelectric, and energy storage properties of the ceramics are studied.The results show that buried sintered ceramic can inhibit the volatilization of Bi 2 O 3 , reduce the generation of oxygen vacancy, make the microstructure more uniform, and reduce the main grain size.The breakdown electric field strength increases from 210 kV/cm to 310 kV/cm.W rec increases from 1.759 J/cm 3 to 4.923 J/cm 3 , which is an increase of 179.9%.It shows good stability in the energy storage temperature and frequency.In this study, the advantages of buried sintered ceramic from the microscopic level to electrical properties are deeply discussed, which provides a new method for further improving the energy storage performance of BNT-based lead-free ferroelectric ceramics and lays a theoretical foundation for the development of BNT-based lead-free ferroelectric ceramics with high storage performance.

Figure 2 .
Figure 2. SEM pictures of (a) ordinary sintered BNT-LFO ceramic and (b) buried sintered BNT-LFO ceramic; the insets are the distributions of the grain size.

Figure 2 .
Figure 2. SEM pictures of (a) ordinary sintered BNT-LFO ceramic and (b) buried sintered BNT-LFO ceramic; the insets are the distributions of the grain size.

Figure 2 .
Figure 2. SEM pictures of (a) ordinary sintered BNT-LFO ceramic and (b) buried sintered BNT-LFO ceramic; the insets are the distributions of the grain size.

Figure 4 .
Figure 4.The bipolar P-E loops of (a) ordinary sintered and (b) buried sintered BNT-LFO ceramics.(c) The bipolar P-E loops of ordinary sintered and buried sintered BNT-LFO ceramics at 150 kV/cm.(d) The variation in the Pmax and Pr values of ordinary sintered and buried sintered BNT-LFO ceramics at 150 kV/cm.The curves of different colors represent the bipolar P-E loops under different electric fields.Due to the different materials a and b, the bipolar P-E loops are different.

Figure 4 .
Figure 4.The bipolar P-E loops of (a) ordinary sintered and (b) buried sintered BNT-LFO ceramics.(c) The bipolar P-E loops of ordinary sintered and buried sintered BNT-LFO ceramics at 150 kV/cm.(d) The variation in the P max and P r values of ordinary sintered and buried sintered BNT-LFO ceramics at 150 kV/cm.The curves of different colors represent the bipolar P-E loops under different electric fields.Due to the different materials a and b, the bipolar P-E loops are different.

Figure 5 .
Figure 5.The unipolar P-E loops of (a) ordinary sintered and (b) buried sintered BNT-LFO ceramics.(c) The unipolar P-E loops of ordinary sintered and buried sintered BNT-LFO ceramics at their maximum electric fields.(d) The variation in the Wrec and η values of ordinary sintered and buried sintered BNT-LFO ceramics.

Figure 5 .
Figure 5.The unipolar P-E loops of (a) ordinary sintered and (b) buried sintered BNT-LFO ceramics.(c) The unipolar P-E loops of ordinary sintered and buried sintered BNT-LFO ceramics at their maximum electric fields.(d) The variation in the W rec and η values of ordinary sintered and buried sintered BNT-LFO ceramics.Materials 2024, 17, x FOR PEER REVIEW 7 of 11

Figure 6 .
Figure 6.A comparison of the Wrec in this work with that of other previously published lead-free BNT-based ceramics [29-52].

Figure 6 .
Figure 6.A comparison of the W rec in this work with that of other previously published lead-free BNT-based ceramics [29-52].

Figure 7 .
Figure 7. (a) The unipolar P-E loops of buried sintered BNT-LFO ceramic under different temperatures.(b) The variation in Wrec and η of buried sintered BNT-LFO ceramic under different temperatures.(c) The unipolar P-E loops of buried sintered BNT-LFO ceramic under different frequencies.(d) The variation in Wrec and η of buried sintered BNT-LFO ceramic under different frequencies.

Figure 7 .
Figure 7. (a) The unipolar P-E loops of buried sintered BNT-LFO ceramic under different temperatures.(b) The variation in W rec and η of buried sintered BNT-LFO ceramic under different temperatures.(c) The unipolar P-E loops of buried sintered BNT-LFO ceramic under different frequencies.(d) The variation in W rec and η of buried sintered BNT-LFO ceramic under different frequencies.

Table 1 .
A comparison of the energy storage ability in this work and other previously published lead-free BNT-based ceramics.